Diesel engines are preferred for many heavy duty applications such as long-haul trucking, for instance, due to performance characteristics inherent in the Otto cycle upon which they are based. Unfortunately, such performance comes with the cost of an increase in certain regulated exhaust constituents. One class of exhaust constituents that must be controlled are oxides of Nitrogen (NOx) and one way of doing so, in automotive applications, is through the use of an active selective catalytic reduction device (“NOx trap”) disposed in the exhaust system of the diesel engine. A typical NOx trap utilizes a catalyst coated substrate that is “activated” through the addition of a diesel exhaust fluid (DEF) that may be injected into the diesel exhaust gas at a location upstream of the trap. The DEF mixes with the diesel exhaust gas and reacts with the catalyst coated substrate in a known manner to reduce certain Oxides of Nitrogen (NOx compounds).
Automotive applications that employ DEF systems for the reduction of NOx may carry a supply of DEF in a tank that is fluidly connected via a supply system to the diesel engine exhaust system. A challenge in the design of these systems is that DEF tends to freeze around 11 degrees centigrade (−11° C.) which is well above the minimum operable temperature of the vehicle. Freezing of the DEF in the DEF tank may be driven in part by the thermal mass of the in-tank DEF pump assembly, resulting in the final fluid portion of the tank residing above or adjacent to the pump assembly. As the final fluid portion freezes, the DEF experiences an expansion rate of about 10% which results in the application of significant forces on the pump assembly. Damage may result.
It is desirable to provide a DEF system that avoids the damaging force that may result when DEF freezing occurs at low operating temperatures.